Optimism over the new malaria vaccine being tested in Africa could be set to fall after results suggested its effect could reduce over time and that it disappears fastest in children who are most exposed to malarial mosquito bites. Scientists point out that they will need to see the full results, from large-scale trials of thousands of children, before they really know how useful the vaccine – developed by GlaxoSmithKline and known only as RTS,S – will be. But the tests follow a string of disappointing results, which have clearly shown how difficult it is to make a vaccine to protect against the disease, which kills more than 650,000 children, most of them very young, a year. Last year a large-scale phase III trial showed that the vaccine saved only a third of babies aged six to 12 weeks old from falling ill. This is the age group that the vaccine is intended for, but the protection was lower than that which appeared in older children. "The efficacy came back lower than we had hoped, but developing a vaccine against a parasite is a very hard thing to do," said Bill Gates, co-founder of the Bill & Melinda Gates Foundation and one of the research funders, at the time. The latest findings, published in the New England Journal of Medicine, come from a smaller phase II trial that began in 2009 and was designed to assess safety and efficacy ahead of the large-scale phase III trials. It involved 447 children in Kilifi, Kenya, 320 of whom were followed up for four years.